Permeability is a critical parameter in determining the productivity of oil-bearing formations. It is conventionally measured by removing samples of the formation through coring and then evaluating the cores in a laboratory. This is a slow and expensive process. In "Synthetic Microseismograms: Logging in Porous Formations", Geophysics, Vol. 39, No. 1 (February 1974), J H Rosenbaum suggested that permeability could be measured acoustically by means of the Stoneley wave, a borehole acoustic mode, using Biot's theory to model the fluid saturated formation. Such measurements of permeability from Stoneley are potentially faster, less expensive, and more thorough than core measurements. An example of such a technique is disclosed in U.S. Pat. No. 4,964,101 ("the '101 patent", incorporated herein by reference) which proposes a model based on Biot's theory relating Stoneley phase slowness to a number of parameters including permeability (mobility). It has been proposed to use this model by conducting calibration measurements in regions of no permeability and known permeability so as to derive a complex dispersion curve. This is compared with complex dispersion curves from the model parameterized by permeability. The permeability of the best-fitting model dispersion curve is the permeability estimate for the formation. Suitable borehole logging tools for performing such measurements can include a monopole source, two dipole sources and an array of receivers and are described in U.S. Pat. Nos. 4,850,450, 5,036,945 and 5,043,952 all of which are incorporated herein by reference.
This approach and others relating permeability to Stoneley slowness often suffer from errors in the estimate of permeability which can sometimes be as high as 260% total relative permeability error. It is an object of the present invention to provide a method which allows this error to be reduced.